It is generally agreed that one difficulty with utilizing fuel cells to power the propulsion system of electric vehicles is the requirement that such vehicles be operable at temperatures below that at which water will freeze. Freezing provides potential mechanical damage as a consequence of the expansion of ice, and presents problems due to the inseparability of water and the fuel cell processes. Heretofore, various methods of initiating operation of a fuel cell, preparatory to the operation of an electric vehicle, have concentrated on providing heat, either by reaction or combustion of fuel, or by means of battery power, to various water and other coolant conduits and reservoirs. Other efforts are directed toward processes designed to accelerate the rate at which a fuel cell stack will heat up to above-freezing temperatures, as a consequence of its own operation.
In copending U.S. patent application Ser. No. 10/390,439 filed Mar. 17, 2003, a PEM fuel cell stack at subfreezing temperature is connected to a vehicle propulsion system or other electric load within a few seconds or as soon as the stack provides open circuit voltage. According to the invention, the fuel cell stack is started with more than a stochiometric flow of fuel and at least stochiometric flow of oxidant, which may be at subfreezing temperatures, or not, whereby to prolong operation without localized heating, thereby permitting the vehicle (or other load) to be used during the time that the apparatus and fluids are being heated to suitable, operational temperatures.
In one known type of PEM fuel cells, the coolant water is managed through porous water transport plates, and by recirculating the water through a restriction, the water is caused to be at between 7 and 21 kPa (1 to 3 psi) below the pressure of the reactants, which are typically at atmospheric pressure. This ensures that water will not pool in the reactant gas channels, that the water is forced into the water channels, and that the hydrophilic substrates will not be flooded and will have sufficient open porosity to permit reactant diffusion. However, when freezing temperatures are encountered, the water in the reactant channels, coolant channels, water pump and other conduits of the water circulatory system is drained upon shutdown of the fuel cell system; upon startup, there is no circulating water so there is no way to maintain negative pressure in the water channels. Therefore, water can build up in the reactant channels.
In one embodiment of said copending application, at least one of the reactant gases is provided to the fuel cell stack at a pressure of at least about 4 kPa (0.6 psi) above the pressure of any water in the water channels, which typically will be about atmospheric pressure. This prevents liquid water from pooling in the reactant channels, and flooding the electrode substrates, which is particularly important in the oxidant gas reactant channel where product water can accumulate.